Rate Matters, SPS (Details) - SPS $ in Millions	1 Months Ended							12 Months Ended
	Apr. 30, 2017 USD ($)	Feb. 28, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 30, 2016 USD ($) MW	Sep. 30, 2016 USD ($)	Feb. 29, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
PUCT Proceeding - Appeal of the Texas 2015 Electric Rate Case Decision
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised Requested Rate Increase							$ 42.1
PUCT Proceeding - Texas 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Revised Requested Rate Increase					$ 61.5
Public Utilities, Requested Rate Increase (Decrease), Amount						$ 71.9
Public Utilities, Requested Rate Increase (Decrease), Percentage						14.40%
Public Utilities, Requested Return on Equity, Percentage						10.25%
Public Utilities, Requested Rate Base, Amount						$ 1,700.0
Public Utilities, Requested Equity Capital Structure, Percentage						53.97%
Public Utilities, Revised requested rate increase, including rate case expenses					$ 65.5
Public Utilities, Total Estimated Impact of Stipulation			$ 51.8
Texas 2016 TCRF Application [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 16.1
Public Utilities, Requested Rate Increase (Decrease), Percentage		1.80%
NMPRC Proceeding - New Mexico 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				$ 41.4
Public Utilities, Requested Rate Increase (Decrease), Percentage				10.90%
Public Utilities, Requested Return on Equity, Percentage				10.10%
Public Utilities, Requested Rate Base, Amount				$ 832.0
Public Utilities, Requested Equity Capital Structure, Percentage				53.97%
Public Utilities, Future Decline in MW Sales from Certain Wholesale Customers included in Rate Case | MW				380
Public Utilities, Capital Expenditures				$ 20.1
Public Utilities, Changes in Allocator, Including Wholesale Load Reductions				11.5
Public Utilities, Transmission Expenses				4.7
Public Utilities, Depreciation, Including Adjustment of Tolk Service Life				3.6
Public Utilities, Rate Case Expenses				1.1
Public Utilities, Other net				0.4
SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Billed Charges For Transmission Service Upgrades				$ 12.8
Public Utility Commission of Texas (PUCT) | PUCT Proceeding - Appeal of the Texas 2015 Electric Rate Case Decision
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Decrease, Net of Rate Case Expenses								$ 4.0
Southwest Power Pool (SPP) | SPP Open Access Transmission Tariff Upgrade Costs
Public Utilities, General Disclosures [Line Items]
Public Utilities, Length of Payment Period Requested, In Years					5 years
Subsequent Event [Member] | PUCT Proceeding - Texas 2016 Electric Rate Case
Public Utilities, General Disclosures [Line Items]
Public Utilities, Mechanism Ensuring Recovery of Revenue Using Final Rates	$ 13.8